Also, as Shelly has pointed-out, the use of a comma can add clarity.
If you insert a comma into a., " They were being disrespectful, talking loudly, and using bad language.", then in the list each behavior stands alone.

A significant difference between a) and b) is that in a) specific behaviours are being described in the second clause. In b)'s second clause, a particular attitude is being described. So in the case of b), the second clause is merely underscoring or amplifying what has been stated in the first.